CONSTRUCTIVE SOLUTIONS TO PROTECT NUCLEAR OBJECTIVES AGAINST
EARTHQUAKES, EXPLOSIONS, SHOCKS AND VIBRATIONS
Authors: Viorel Serban, Adrian Panait, Marian Androne, George Ciocan, Ioana Florea- SITON - Subsidiary of
Technology and Engineering for Nuclear Projects
Abstract
The paper is a presentation of a constructive solution for to protect nuclear objectives
against earthquakes, explosion, shocks and vibration employing new SERB 800x800 devices
developed by SITON.
The new SERB 800x800 devices can overtake permanent compression loads (up to 6000
KN) over which dynamic compression and thrust loads up to 1500 KN may overlap. On vertical
direction, the devices have a very high stiffness in order to avoid the occurrence of different
settlements that may lead to additional loads in the isolated supra-structure. On horizontal plane, the
devices are sliding with a very low friction coefficient (ranging between 0.03 – 0.08) allowing
displacements up to ± 225 mm while limiting the range through a non-linear stiffness which starts
operating after 2/3 of the displacement has consumed.
The advantages of the isolation against dynamic actions such as earthquakes, explosions,
shocks, vibrations and plane crash impact with nuclear objectives are presented for one PHWR 700
NPP standard unit but the solution may also be applied to any type of nuclear objective including
detritiation installation.
1.
INTRODUCTION
A safe, easy to apply and economically advantageous method to increase the safety margin
of nuclear objectives against dynamic actions is their isolation against the foundation ground. For
example, with CANDU PHWR and ACR type Nuclear Power Plants (NPP) the nuclear part or only
the Reactor Building (R/B) or Spent Fuel Bay Building (SFB) may be isolated.
The paper presents the advantages and disadvantages of SERB-SITON solution for the
seismic isolation of the reactor building (R/B) and spent fuel bay building (SFB) for a site similar to
Cernavoda Site.
The presented method may be applied to any type of stiff or flexible building and it provides
the protection of the objective against any dynamic action coming from the outer environment and
manifesting like a common cause event.
For to substantiate the seismic isolation solution this document presents alternative analyses
regarding the seismic behavior of isolated buildings considering an earthquake-type dynamic action
with the Design Response Spectrum typical to Cernavoda NPP and the sinusoidal type time history
amplitude accelerations and repetition periods equivalent to the design seismic movement.
The analyses of the alternatives have considered two modes of the isolation system
behavior: oscillating system and non -oscillating system, also taking into account the Coulombian
type gliding dynamic friction.
The main target in the protection of an objective against external dynamic actions, i.e.
earthquake type, shocks, vibrations, etc. is that the objective be so stiffened that the dynamic action
should not be transferred to the protected objective or only a small quantity of the energy be
transferred, on each cycle of the action (oscillation cycle). Moreover, the oscillating system
composed of the objective building and the foundation ground, needs also to be surveyed so to
avoid the accumulation of kinetic and potential energy capable to generate an amplification of the
objective dynamic response following to the external dynamic action.
The most advantageous method to protect the objectives against dynamic actions is the
objective isolation against the ground by a non-oscillating mechanical system that is capable to cutoff the transfer of the dynamic action between the objective building and the foundation ground.
For to apply such a solution, usually the objective is divided into 2 modules on vertical, as
follows:
1
-
lower module, called infra-structure, usually including the foundation and the basement
or the basement, foundation and ground floor;
- upper module including all the levels of the building above the separation surface.
Between the two modules, a mechanical system capable to overtake the permanent loads and
the dynamic actions (without transferring and amplifying them) under safety and stability
conditions for the objective as a whole assembly.
2.
DESCRIPTION OF STRUCTURES. REACTOR BUILDING (R/B)
The Reactor Building is a reinforced and prestressed concrete structure consisting of the
containment and the inner structures supported on the base slab. The inner structures are made of 3
sub-structures: calandria vault, fuel transfer structure and the inner structure itself.
The Containment Building (Fig. 1) is a prestressed concrete structure, cylindrical in shape,
with two domes on top. Its diameter is 41.5 m, the wall thickness is 1.57 m and the high is about 52
m. The clearance between the two spherical domes is filled in with cooling water.
The Reactor Building inner structures are made of thick reinforced concrete walls arranged
as per axes A.B.C.D and E (on B-D direction) and axes 21, 22, 23, 26 (on A-C direction), four (4)
reinforced concrete floors at El. 100, 107, 112, 117 a base slab and sub-base between El. 88,7 m and
El.93.9 m (Fig. 2). The base of the Reactor Building foundation is located at about 88.7 m (4.78
NMB).
The Reactor Building weight is about 880000 KN out of which the sub base-base slab
represents 240000Kn. Besides its function to provide the climatic protection of the systems,
equipment and operation personnel in case of a coolant loss (Design Base Accident-DBA), the
containment is also providing the essential function of retaining the accidental radioactive releases
from the process systems and equipment in various emergency cases, including DBA, since the
containment is a special protection measure for the population and the environment.
The Containment is designed to withstand and maintain its tightness degree at a pressure
resulted from a Design Base Accident (DBA) with the Dousing System in service. The Containment
is also designed to maintain its integrity and tightness at a DBE earthquake. For to provide the
tightness, the inner side of the containment wall is epoxy lined.
3.
CERNAVODA
NPP
TRADITIONAL
SEISMIC
QUALIFICATION
For to overtake in safe
conditions, the seismic loads generated
by earthquakes likely to occur on
Cernavoda Site and having accelerations
smaller or equal to a DBE (0.2 g on any
direction in horizontal plane), the
buildings, equipment, piping systems,
Fig. 1. Reactor Bldg. - General View
Fig. 2. Reactor Bldg. - Cross-Section
etc. were differently sized as per the type
El.93 m
of structure and material so that the total
stresses resulted from the groups of loads including an earthquake, should not exceed the stresses
associated to the yielding strength of the ductile materials and cracking should not occur with
brittle materials.
The stress and distortion condition in buildings, equipment and components of the Plant
were determined with detailed dynamic models in which the possibility of amplifying the dynamic
response of the analyzed system due to the kinetic and potential energy built-up during a seismic
movement, was considered.
The traditional seismic qualification of CANDU type NPPs is based on the concept of
minimizing the amplification of the seismic response of buildings, equipment, piping systems, etc.
by stiffening them within the allowable technical limits.
23
A
21
D
1
24
25
22
26
27
B
C
R018
2
R006
17
R010
ER
19
14
18
R014
R004
R008
DR
R109
R001
R017
R012
CR
9
R011
8
R003
7
13
12
BR
6
10
R007
R013
5
11
Emergency Airlock
3
14
15
4
16
AR
R005
R009
10
11
12
13
14
15
16
17
18
19
Bazin stropire
Armături sistem de stropire
Pompă moderator
Schimbător căldură moderator
Cabineţi izolare fideri
Faţa reactorului
Reactor
Mecanisme reactivitate
Pompă circuit primar
Pod MID
1
2
3
4
5
6
7
8
9
Cărucior MID
Ansamblu Catenaria MID
Poziţie întreţinere MID
Uşa camera MID
Rezervor întârziere sistem răcire protecţie biologică
Răcire cheson calandria
Presurizor
Generator abur
Pod rulant generator abur
2
4.
SEISMIC ISOLATION OF R/B AND SFB
The solution is recommended to be applied to important objectives such as the detritiation
installation, radioactive waste storages, etc. With CANDU type NPPs, the solution consists in the
elimination of the sub-base, of the radial, central and peripheric keys between the base slab and subbase as well as of the membrane between the base slab and sub-base. The R/B is supported on 270
isolators (SERB 800x800 type fig. 3-4) installed between the base slab and a reinforced concrete
plate. Since between the R/B and SFB building there are technological connections required by the
fuel channel transfer and the equipment and material airlock support, the seismic isolation system
must be so constructed to provide the connection to R/B and allow for no relative movement during
an earthquake, a relative movement that might negatively affect the fuel channel transfer and the
equipment and material airlock. The SFB building shall rest on 40 isolators.
Fig. 5-6 illustrates an alternative for the installation of SERB 800x800 isolation devices
beneath the R/B and SFB building and of SERB-B-204 telescopic devices (1, 2, 8).
Considering that with the building isolated solution the seismic loads on R/B and SFB
building (including sites with high seismicity), are small, the proposal consists in the analysis of the
solution to raise the foundation elevation from El. + 80.75 by about 5 m while correspondingly
modifying the technologic connections between the R/B, Service Building (S/B0 and Nuclear
Auxiliary Building.
Fig. 3. SERB 800 x 800 prototype. 0 and 150mm displacement
Fig. 4. Experimental hysteresis characteristic
In case the ground structure at the new foundation elevation allows the overtaking of
permanent loads and seismic loads with the isolated solution, the only problem detrimental to the
raise of the foundation elevation, is represented by the pressure losses along the live steam, supply
water and condensate return piping. Bearing in mind that the pressure losses along these pipes, is
increasing by maximum 5%, such a loss is not causing a major quantifying power loss in the steam
turbine and practically, neither an additional power supply to the circulating water pumps.
5.
ADVANTAGES AND DISADVANTAGES OF A BUILDING ISOLATION AGAINST
DYNAMIC ACTIONS
For to highlight the advantages of SERB-SITON solution for building isolation, a concrete
case is analyzed, namely, the seismic isolation of the reactor building and of the Spent Fuel Bay
building.
The analyses are conducted for a DBE but the solution may be applied for any building in
order to reduce the effect of any dynamic action that is likely to negatively affect the building.
The isolation of a building employing SERB 800 X 800 devices can be carried-out at any
level of the building but it is recommended to do it at the ground floor level for to ease the
construction procedure.
The buildings isolated by SERB – SITON solution make –up a non-oscillating system with
relative displacements small or equal to the maximum displacement of the dynamic actionand
important dis-amplification of the dynamic action.
Herein below it is a presentation of the technical advantages for the seismic isolation of the
reactor building and Spent Fuel Bay building at a PHWR 700 NPP, followed by a presentation of
the method.
3
5.1 Overall advantages
1. The reduction of the seismic action applied to the building, equipment, piping systems, by
about 4 times for equipment and piping systems located in the lower part of the building, and
by up to 40 times for equipment and piping systems located on the upper part of the building
in the case with the building isolated (compared with the unisolated building solution);
2. The reduction of the probability to \ radiologically pollute the environment due to the fact that
the isolation system allows an efficient and direct (visual) monitoring of the R/B and SFB
Bldg. foundations.
3. A better overtaking of the shocks generated by the impact with airplanes.
5.2 Advantages for the NPP – BOP (Civil Part).
1. The removal of a 3.68 m thickness from the reinforced concrete sub-base, of the radial and
peripheric keys. The volume of concrete is reduced by about 8000 m3 and the quantity of rebars by about 360 tons.
2. The reduction of the sectional seismic stress in the R/B and SFB structure by 5-10 times
allowing thus a reduction of the reinforcement material by at least 15%. Such a reduction is
benefic both in point of material savings ( by about 592 tons) and in point of facilitating and
improving the quality of the reinforced concrete structure ( i.e today, due to the large number
of reinforcing bars a compact concrete pouring is difficult and segregation and cavities in the
reinforced concrete structures are produced).
3. The process of prestressing the perimetral wall and the containment base slab is facilitated
and the time is shortened. The elimination of the membrane between the sub-base the base
slab, of the central, peripheric and radial keys between the sub-base and the base slab.
4. The possibility to make a prestressed concrete infrastructure to the SFB Bldg will provide a
very good waterproofing. In this case the inner lining of the SFB walls by stainless steel
plates welded onto the supporting steel structure, is no longer required and the savings of
material (stainless steel) is about 260 tons.
5.3 Advantages for the NPP – NSP (Process Part)
1. The reduction of the seismic action on the equipment and piping systems results in the
elimination of the seismic qualification requirement for equipment and piping systems in the
R/B and SFB Building. In other words such a reduction will represent at least 1% savings
with the equipment purchase and the elimination of snubbers in the piping systems and
equipment - a total of 370 snubbers estimated to 3700000 Euro. In case that a decision is
taken to keep on some steam generator snubbers for safety considerations, they may be
substituted with smaller snubbers or other types of supports.
2. The overall reduction of the equipment cost is estimated to 1.72 million Euro considering that
the cost of the installed equipment is about 172 million Euro for one CANDU PHWR
700MW unit.
5.4 Disadvantages of the isolation solution for NPP – Civil Part (BOP)
1. The construction of a full large excavation of about 1500 m3 than with the classic solution.
2. The construction of about 6800 m3 reinforced concrete poured in the base plate and the
supporting wall and the stands for the isolation devices. The re-bars for reinforcing the
concrete base plate and the supporting wall and the stands is estimated to amount 580 tons.
5.5
Disadvantages for the NPP Process Part (NSP)
The seismic isolation of R/B and SFB Building will lead to a relative movement of
maximum ±20 cm on any direction in horizontal plane between these buildings and the other
buildings in the plant. In this case the connecting process piping of the steam circuit, supply water
system, condensate return, emergency cooling water, etc need to be capable to overtake the relative
movement between the isolated buildings and the un-isolated buildings in safe conditions. To
overtake the seismic loads, the supports on R/B and S/B need to be so constructed to allow the
4
relative movement in the horizontal plane coincident with the displacements from thermal
expansions. The spatial development of the piping systems allow the overtaking of relative seismic
displacements between the seismically isolated and unisolated buildings while SERB type supports
can provide the overtaking of displacements from thermal expansions.
The seismic isolation is not involving important modifications of the electric and I&C cable
systems. The only modification consists in the extended length of such cables by about 0.5 m and
the construction of some racks to allow ± 200 mm relative movements in horizontal plane.
6.
ESTIMATION OF COSTS RELATED TO THE R/B AND SFB SEISMIC
ISOLATION.
1. The fulfillment of the R/B and SFB isolation system implies costs related to purchasing,
amounting to about 7000 euro/piece and to installation works amounting to 1000 euro/piece.
The total amount required would be 310 X 8000 Euro = 2480000 Euro.
2. The volume of poured concrete is reduced from 8000 m3 representing the eliminated 3.62 m
thick sub-base, down to 6800 m3 representing the 1.6 m thick base plate – general base slab,
the 0.5 m supporting wall and the isolation device stand. The savings in respect of concrete
quantity is 1200 m3 amounting about 200 Euro/m3 X 1200 m3 = 240000 Euro.
3. The additional quantity of reinforcement required for to construct the base plate and the
supporting wall is 580 tons – 380 tons = 280 tons representing an additional cost of 2000
Euro/ton X 200 tons = 400000 Euro.
4. The less quantity of reinforcement for R/b as a result of reducing the seismic loads is about 592
tons representing savings : 2000 Euro/ton X about 592 tons = 1184000 Euro.
5. The elimination of the snubbers on the live steam piping represents 3700000 Euro savings.
6. The reduction of costs associated to the equipment and system seismic qualification because of
the reduction of the seismic accelerations is estimated to 1.72 million Euro.
The R/B and SFB seismic isolation by SERB-SITON solution is generating a cost saving of
about 154000 Euro. In case that the solution to raise the R/B and SFB by 5 m is accepted, the
isolation solution becomes even less expensive than the classic solution and financially more
attractive. Note that the figures are for information only.
7.
DESCRIPTION OF THE ISOLATION SOLUTION.
The R/B is a stiff structure made of prestressed reinforced concrete (the containment and the
base slab) and unstressed concrete (inner structures and the sub-base). The SFB building is made of
prestressed reinforced concrete and it is technologically connected to the R/B via the fuel transfer
channel.
In point of construction The R/B is not connected to S/B, Nuclear Auxilliary Bldg., and
BOP but it is technologically connected via the live steam, supply water, emergency cooling water
piping and electric and I&C cable systems. Also, for the equipment transfer, the R/B is provided
with the equipment airlock that is supported on the concrete structure of the fuel transfer channels to
the SFB and the personnel access the containment is provided with a personnel airlock supported on
the building base slab. The spent fuel transfer from the R/B to the SFB is made via a technologic
connection that must be tight, with zero water leakage from the fuel transfer channel.
Between the two structures – R/B and SFB (fuel transfer channels)– there must be no
relative movement greater than the elastic distortion in the joints between the two
structures(buildings) so to prevent water leakage from the fuel transfer channel .
The R/B seismic isolation need to be so done that the technological connections with the
other buildings should not be affected. For that reason the R/B isolation (see Fig 4-5) is proposed to
be as follows:
1. The construction of a general 1.6 m thick reinforced concrete sub-base embedded into the
ground at El. + 88.75 m. The sub-base surface should be 32000 m2 spread beneath the R/B
and SFB and to the outside with 1.0 m. In the area of SFB and R/B the sub-base is extended
for to make connections between the base slab of the two buildings that provide a “zero”
relative movement between the buildings and the fuel transfer channel.
5
2. The construction of a 0.5 m thick and 9.65 m high supporting wall on the sub-base boundary
at El. + 90.35 m and El. + 100 m.
3. The construction of 310 square-shaped (1.5 m side X 1.0 m high) reinforced concrete stands
to embed SERB 800 X 800 isolation devices. The distribution of shocks beneath the sub-base
is so made that the R/B and SFB structure weight loads are quite uniformly distributed on
SERB isolation devices. In the area associated to the R/B inner structures 136 stands are
constructed, 138 stands for the perimetral wall area and 36 stands for SFB.
4. The installation of SERB 800 X 800 isolation devices on stands is made by 12 X M32 bolts in
the embedded parts in the reinforced concrete stands.
5. The installation of the upper anchoring parts and their attachment to SERB 800 X 800 devices
is made by 12 X M 32 bolts that are embedded in the reinforced concrete caps sized: 1.5m X
1.5 m X 0.4 m.
6. The construction of the R/B and SFB building base slab on the associated caps is made with
no restraints in respect of the employed construction technology.
7. The construction of a connection between the R/B base slab and SFB base slab in the area of
the fuel transfer channels for to avoid relative displacements between the two base slabs. The
proposal is to construct 3 spatial metallic shaped connections of 2 X 2 m sizes in plane
section.
The R/B and SFB isolation employing SERB 800 X 800 devices offers the following
advantages:
1. The possible seismic acceleration of R/B and SFB is ranging between 0.05 – 0.10 g for any
earthquake irrespective of the ground acceleration if the ground seismic movement is smaller
than ± 180 mm. If the ground seismic movements are greater than 180 mm, the seismic
accelerations transferred to the buildings will exceed the above values dependant on the
difference between the ground seismic movement and the relative distortion allowed by
SERB devices;
2. The reduction of the reinforcement percentage in R/B and SFB, considering that the sectional
seismic loads and stresses in the structural elements of R/B and SFB are reduced by at least 7
times, which allows a reduction of the reinforcement percentage and re-bars of at least 15%
for a site with the ground maximum acceleration of 0.2 g for a DBE.
3. The elimination of the metal plates on the lower front side of the containment in the area of
maximum seismic stress considering the low seismic stresses in the R/B containment with the
isolation alternatives.
4. The elimination of the 6 mm thick stainless steel lining on the inner side of SFB because with
the isolated solution the SFB may be constructed of post-stressed reinforced concrete which
prevent the possibility of water leakage from the bay.
The reduction of the number or even the elimination of snubbers on the live steam piping in
R/B.
All the equipment in R/B and SFB building need not be qualified at seismic accelerations
and thus the cost is decreased by at least 5%.
One SERB 800 X 800 device consists of a lower case and an upper case which are
practically identical in point of construction and functionality
The two cases are interconnected by a central device offering relative independent
movements between the two cases and elastic limiting and damping of the maximum displacement,
at pre-set values.
6
+100.00
+100.00
CONTRAFORT
+93.83
+92.15
SUBRADIER
+90.35
+88.75
Fig 5 – Isolators arrangement – R/B and SFB building
Fig 6 – Detail on excavation and device installation.
8.
MATHEMATICAL EVALUATION OF THE SEISMIC ISOLATION EFFECTS
The isolation of a building assumes the construction of at least one mechanism at a lower
level of the building aimed to cut-off the transfer between the two parts of the building: the
infrastructure and the suprastructure. At present, most of the isolation systems are elastic type. They
show the disadvantage that they may build-up potential energy at each oscillation cycle of the
dynamic action. In case that the proper capacity to dissipate the built-up energy is not provided,
amplifications of the dynamic action in displacements may occur because the isolated
suprastructure vibration period is large. During an earthquake the suprastructure maximum
displacement is about 3 times greater than the maximum displacement of the foundation ground.
The isolation system developed by SITON is a non-oscillating system with gliding and low
friction which makes the suprastructure, isolated together with the isolation system, avoid
generating (making-up) an oscillating system which is building-up energy.
In this case the suprastructure displacement is actually zero and the relative displacement of
the isolation system is below the ground maximum displacement during an earthquake.
The isolation system developed by SITON may provide a relative displacement of about 8
times bigger than the maximum seismic displacement (design seismic displacement) of the ground
on Cernavoda Site and of about 1,5 times greater than the maximum seismic displacement of the
ground in Bucharest area, generated by an earthquake like the one in 1977.
For to avoid the effect of shocks produced by exceeding the limit of relative displacement of
the isolation system developed by SITON (a case difficult to imagine with earthquakes likely to be
produced in Romania), the isolation system is provided with elastic non-linear limitators which start
operating after a relative displacement meaning two thirds of the maximum displacement of the
devices has been consumed.
Following to the SERB-SITON isolation system sizing analyses, the isolation system can be
accomplished for two cases:
- an oscillating isolation system ( a theoretical case for to see wether the system would
provide the relative displacements for this case);
- a non-oscillating isolation system (with gliding-induced friction).
According to the design standards, the seismic isolation principle assumes that the isolation
system vibration period – isolated building, should be at least 3 times greater than the period of the
building anchored at its base ( embedded in the ground) for the building in the isolated solution,
behave rigid versus the seismic movement(action) filtered by the seismic isolation system (3).
Besides, for to protect the isolation device it is desirable that the device vibration period be at least
50% greater than the corner period (Tc) of the response spectrum in the site area.
In this case, dynamically speaking, the seismically isolated building will behave like a rigid
body elastically supported and the relative displacement in the isolation devices will make the
seismic energy be only partially transferred to the isolated building (5-6).
Fig. 7 illustrate the time history accelerations and specific response spectra of the absolute
accelerations and the relative displacements associated to the time history accelerations for
Cernavoda NPP Site.
7
Fig. 7. Design time-history accelerations for Cernavoda NPP Site, response spectrum of accelerations for various
damping and response spectrum of the relative displacements for various damping.
β = 5%, 10%, 20%, 30%, 40%, 50%
The establishment of the parameters for sizing the seismic isolation devices for buildings shall
develop in two alternatives:
Alternative 1 – The device with the isolated building make-up an oscillating system capable to
accumulate kinetic energy for the building and the elastic potential in the isolation device generated
by the seismic movement. In this case, the stiffness and damping of the isolation system (isolation
device and limiting and reverting telescopic device) shall be evaluated so that the relative
displacement in the system be maximum 18 cm (maximum allowable displacement in the SERB
800 X 800 isolation device prototype) for a ground acceleration of 0.2 g (maximum design
acceleration at Cernavoda NPP) on a direction in the horizontal plane.
Alternative 2 – The device with the isolated building make-up a quasi-oscillating system. The
isolated building may move freely with friction on a distance imposed by the isolation system.
After spending the free movement (between 1/2 and 2/3 of the maximum distance) the building is
non-linear moved in order to limit its displacement and make it revert to the initial position.
8.1. Alternative 1
For analyses, the response spectrum of the relative displacements is used ( Fig 7 calculated
by SAP 2000 on basis of the design response spectrum of the accelerations).
The horizontal line points out the maximum allowable relative displacement of the isolation
device(0 = 18cm). The design criterium is that the relative displacement in the response spectrum
be under the maximum design displacement, 0. Since the relative displacement depends on the
eagen period of the isolation device – building assembly and the effective damping ratio, the
analysis need to set-up optimum values for the eagen oscillation period and the damping ratio.
The R/B isolation system is made by SERB 800 X 800 devices for to overtake the weight
and by SERB –204 telescopic devices for the control of the building revert to the initial balance
position. The actual viscous damping ratio, e,, is calculated considering the dynamic friction
between the isolation element, , and the viscous damping of the elastic revert elements inside the
isolation device and the telescopic devices for the R/B revert .
The viscous damping energy and the energy due to friction , calculated for one oscillation
cycle, is assimilated like the equivalent viscous damping energy on the same oscillation cycle :
2g
g
   (1  2 ) 2 (2)
( mg  2mx )dx   2 e mxdx (1) it results:  e   e1   e 2 

 0
 0
ciclu
ciclu
where: = dynamic friction ratio between the gliding elements ( PTH on steel  ~ 0.05-0.1.);
= viscous damping ratio specific for the inner springs and telescopic elements, ~5% – 20%; 0=
maximum design displacement 0 = 18cm; =maximum angular frequency of the ground seismic
2
movement  
, Tc=0.7s, [3]); ω = angular frequency of the R/B – isolation device
Tc
2
assembly  
, Teff>2Tc=1.4s
Teff
Selecting a vibration period Teff ~ 3s for the isolation assembly, Fig 7 shows the result: the
equivalent damping ratio need to be minimum e ~ 20%. That means a minimum friction ratio  ~
0.05 and minimum viscous damping  ~ 5%, so, practically, any realistic range of damping/friction
8
ratio. So, an equivalent high damping leads to the decrease of the response spectrum of the relative
displacements, its value falling below the design displacement. The absolute acceleration of the
mass becomes A = 0.84 m/s2 times below the ground maximum acceleration (Amax= 2m/s2).
The total estimated mass of the R/B including the inner structures, the containment and the
base slab, is M= 63710 tons. The total number of isolation devices that support the R/B in the
proposed isolated alternative is Nd= 270 isolation devices arranged beneath the containment
perimetral wall and the inner structure walls and in their vicinity so that a relative uniform relief of
the self-weight is obtained without generating large stresses in the base slab and walls.
For to obtain a vibration period Teff  3s , the stiffness average value of a telescopic device
shall be about keff = 20.6 for about 5% viscous damping (or greater) in function of the selected
friction ratio so that the equivalent viscous damping ratio should not exceed 28%.
8.2. Alternative 2
A. Response of the isolation system with friction
In the beginning, consider that at the isolation interface, the building is moving with
friction since the movement is not limited by elastic-with- damping devices. The aim is to estimate
the nature of the system movement only under friction and displacement conditions.
The law of movement is given by: x  u  g  sign (x )
(3) where: x= relative
displacement; x = relative velocity; u = ground acceleration; = friction ratio at the isolation
interface.
For the case of a sinusoidal acceleration time history of the ground movement having the
amplitude : Ag = 2 m/s2 and fundamental period Ts= 1 s, the ground acceleration is
2
u(t )  Ag sin
t (4)
Ts
In case of a null friction ratio, the building relative movement becomes equal and contrary to
the ground movement and the total displacement is zero, the building staying at rest. Consequently
the total acceleration is also null. In case of a friction ratio other than null, the building relative
displacement is in anti-phase with the ground relative movement but it decreases and it is translated
behind , proportionally with the value of the friction ratio. But the total displacement is increasing
with the increase of the friction ratio but remains at small values for the usual ratios,  = 0.05 – 0.1
(Fig. 8). Calculated values of the building total acceleration gets values corresponding to friction
a f  g equal with 0.5 m/s2, 1.0 m/s2 respectively).
Thus, for the isolation systems acting with gliding, without external forces (usually elastic
forces) within a limited displacement range, the use of rather small friction ratio is maintaining the
displacement at values smaller than the ground movement values. After an imposed movement,
elastic forces may interfere meaning that, on one hand, they may limit the system gliding and, on
the other, they may bring-back the system to its initial position. Such isolation solutions lead to the
cutting-off of the acceleration response down to values corresponding to friction, a f  g .
Relative Displacement Response Spectra for different damping
ratio.
5%
10%
20%
30%
40%
50%
S0 = 18 cm
Relative Displacement
Response Spectra, [cm]
60
50
40
30
20
10
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
Period T, [s]
Fig. 8. Response spectrum of relative
movements for different damping.
Fig. 9. In-time variation of the ground movement and response in relative and total
movements under the action of a sinusoidal type acceleration,  = 0.05,  = 0.1
It is found that the maximum relative movement in case of a friction ratio  = 0.05 – 0.1 is
about 6 cm, much under the characteristic values an isolation device can have. For that reason, the
9
radial telescopic devices arranged on the R/B base slab circumference having the rile to limit and
revert, may have a special characteristic in which along a movement (displacement) of ±3-4 cm
they do not act and than the stiffness of the devices is slowly increasing with the distortion.
B. Response of the isolation system with friction and non-linear displacement limitation.
A numerical estimation of the response of a system provided with a seismic isolation
device with friction and a limiting and reverting telescopic device having a non-linear stiffness
characteristic and viscous damping  = 5% (damping considered constant) , was conducted.
Fig. 11 illustrates the relative displacements of the system for the stiffness characteristic
presented in Fig. 10.
When friction is absent, the
maximum relative displacement is
limited to the 6 cm value (a little
amplified as to the ground maximum
movement of 5 cm) since the braces
are slightly limiting the system
relative displacement.
When friction is present with 
= 0.1, the relative displacement is
Fig. 10. Stiffness-displacement caracteristics
Fig. 11. Time variation of relative
reduced to 4.3 cm. When friction is
for the limiting and recurrence crossdisplacements for an oscilant system
absent, the absolute acceleration is
bracing.
with telescopic devices  = 0, and 0.1.
limited to 0.5m/s2, the stiffness of the
braces limiting (practically without a shock) the relative displacement. When friction is present
with  = 0.1., the absolute acceleration is 1 m/s2, a value resulted from the action of the ground on
the building via friction.
All these analyses conclude that the presence of friction is limiting the relative displacement
and the absolute acceleration both in case of resonance and out of it. The seismic isolation solution
proposed in the paper, is combining the phenomenon of seismic isolation due to the moving-away
of the vibration period ( to a ratio of 3 as to the eagen period of the oscillating system - embedded
into the ground- and as to the fundamental oscillation period of the ground) with the friction
phenomenon for to limit the relative movement. The absolute accelerations are much reduced,
under the value of the maximum ground acceleration. The use of the telescopic devices with
obvious non-linear characteristic and strengthening, allows both the limitation of the random
maximum relative displacements and shocks that may occur during an earthquake, and the system
revert (even partially) to the initial condition before the earthquake.
The computations were conducted without considering the short moments in which the mass
is rigidly (without gliding) engaged by the ground. In case of some friction ratios considered high
for the isolation phenomenon, ( > 0.1) such a rigid engagement have a unfavorable effect.
Therefore it is desirable that the friction ratio be as small as possible ( ~ 0.03-0.07) so to provide
gliding within the longest possible displacement range associated to an oscillation cycle.
Load-displacement characteristic
Load
Bilinear
1.3E+07
1.2E+07
1.0E+07
9.1E+06
Load, [N]
7.8E+06
6.5E+06
5.2E+06
3.9E+06
2.6E+06
1.3E+06
-0.10
-0.08
-0.06
-0.04
0.0E+00
-0.02 0.00
-1.3E+06
0.02
0.04
0.06
0.08
0.10
Displacem ent, [m ]
9.
CONCLUSIONS
The paper is presenting a constructive solution for to protect nuclear objectives against
external actions such as earthquakes, shocks, vibrations, etc, by their isolation employing SERB
devices developed by SITON.
Compared with the isolation systems applied today, SERB-SITON isolation system shows
the advantage that it is a non-oscillating system that can also overtake traction forces and can
elastically limit the relative displacements within the devices in case that the displacement imposed
by the dynamic action is grater than the isolation system displacement.
The isolation system developed by SITON may be applied to any nuclear objective such as a
detritiation installation, radioactive waste storage/disposal, new CANDU type PHWR NPPs amd
ACR NPPs.
10
In the paper the analyses have been conducted for the seismic isolation of the reactor
building and the Spent Fuel Bay building in a PHWR 700 type NPP.
The new proposed solution for the seismic isolation shows technical and economic
advantages and it also shortens the period of construction. All the systems, equipment and piping
systems in the two buildings need not be seismically qualified because the seismic acceleration
transferred to the two buildings with SERB –SITON isolation system, is much below 0.1 g
(function of the dynamic friction ratio during gliding) on any direction in horizontal plane,
irrespective of the seismic acceleration on site.
For the substantiation of the isolation solution the analyses were based on seismic movement
input data from Cernavoda N.P.P. Site design data. Since a maximum relative displacement of 180
mm was imposed, sizing of the seismic isolation devices was carried-out. (stiffness and equivalent
damping).
Following to the theoretical and numerical analyses, also considering the gliding – induced
friction, the results showed that the maximum relative displacement between the building,
seismically isolated by SERB 800 x 800 devices, and the foundation ground is maximum 100 mm
and the maximum seismic acceleration transferred to the buildings is 0.05 g in case that the friction
ratio between the PTFE and stainless steel surface is 0.05.
The friction coefficient experimentally determined on a 1/1 scale model was 0.03g.
REFERENCES
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structure and a network of devices for to overtake and damp so to control the behavior of
buildings, equipment and pipes network under loads conditions ;
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to overtake static and dynamic loads;
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